1. Field of the Invention
The present invention relates to a process for producing a rubber compound charged with silica and more particularly to a method for controlling the process parameters during the mechanical processing of this compound in a closed batch mixer.
2. Description of the Related Art
Hereinbelow, the expression “closed batch mixer” is intended to denote a device comprising a closed container inside which is housed a pair of rotors rotating in opposite directions, to mix the various ingredients of a compound. The device further comprises a pneumatic cylinder located at the top of the container, the piston of which is driven either upwards to open the container and allow the introduction of the ingredients of the compound from suitable filling hoppers, or downwards to exert a pressure on the material being processed in the container, above the pair of rotors.
A pneumatic system placed at the bottom of the container allows the compound to be emptied out at the end of the processing cycle, via the opening of a suitable outlet.
Devices of the type mentioned above have been known for some time: in particular, there are devices of the “Banbury®” type which process the material by means of a pair of tangential rotors, while different devices known under the name “Intermix®” process the material by means of a pair of intermeshing rotors.
A rubber compound which can be prepared by the process of the invention is of the type comprising an unsaturated-chain polymer base which can be hot-crosslinked with sulphur, which are added to at least one silica filler and a silica-binding agent containing at least one sulphur atom.
In the description hereinbelow and in the claims, the term “crosslinkable unsaturated-chain polymer base” is intended to denote any natural or synthetic, non-crosslinked polymer, or mixture of polymers, capable of acquiring all the typical physicochemical and mechanical properties of elastomers after crosslinking (vulcanization) with sulphur-based systems.
In the description hereinbelow and in the subsequent claims, the term “silica-based reinforcing filler” is intended to denote a reinforcing agent based on silicon dioxide (silica), silicates and mixtures thereof, with a surface area, measured by the BET method, of between 80 and 220 m2/g, preferably between 160 and 180 m2/g.
As is known, compounds reinforced with silica fillers are especially used in the production of semi-finished rubbers for tyres, in particular tread bands, since these compounds allow a reduction in the tyre's resistance to rolling when the silica is chemically bonded to the polymer base following subsequent reactions with a binder made of a silane.
The present invention is based on the perception of the problem of the uniformity and reproducibility of the mechanical and rheometric properties of a compound produced for finite and sequential amounts, as being dependent on the values of certain process parameters, which can vary continuously during the process.
In general, it is thought that the difficulties in processing silica-charged compounds are based on the fact that good homogenization of the silica in the polymer would require energetic mechanical processing, with the consequent development of high temperatures, whereas the reaction of silane with the polymer and with silica should be carried out under more controlled temperatures so as not to initiate premature crosslinking of the silane.
Hereinbelow, the material being processed during the various compounding and cooling phases will be referred to as the blend, while the blend discharged at the end of the processing cycle, generally after also adding the vulcanizing system, will be referred to hereinbelow as the compound.
A general process for the mechanical processing of a rubber compound with silica and silane is given in the publication “Silica based tread compounds: Background and performances”, page 14, Table IV, published by Degussa on the occasion of the Tyretech '96 conference held at Bále on 28-29 Oct. 1993.
According to this process, the silica and the silane are simultaneously loaded into the compound being processed, which is blended while keeping the processing temperature below 160-165° C., in order to avoid premature crosslinking of the silane by exceeding this temperature.
U.S. Pat. No. 5,227,425 describes a process for producing tread bands obtained by mixing a polymer base with a high content of silica and addition of a silane. The polymer base is formed from a diene conjugated with a vinylaromatic compound: the diene has a content of vinyl groups of between 5% and 50%.
The polymer base and the silica are processed mechanically in a closed batch mixer, or in an extruder, until a temperature of at least 130° C. is reached, and preferably between 145° C. and 180° C., but not exceeding 180° C.
According to another process, the polymer and the silica are subjected to mechanical processing in two separate phases separated by a cooling phase.
In the first phase of this latter process, the polymer base, the silica and the binder are subjected to mechanical processing until a temperature of greater than 145° C. is reached, preferably between 145 and 170° C.
After removing this blend from the mixer, it is cooled to a temperature below 100° C. and preferably not above 60° C.; this is then followed by a second phase of mechanical processing in a batch mixer together with other ingredients, excluding the vulcanization agents, until a temperature of between 145 and 170° C. is again reached.
The vulcanizing system is then added to the blend by means of a final phase of mechanical processing, in an open two-cylinder mixer, at a temperature below 100° C.
Italian patent application No. 95IT-MI000359A from the Applicant describes a process in which, for the purpose of improving the dispersion of the silica in the polymer base, the polymer base is first mixed with the silica in a closed batch mixer until a temperature of between 165° and 180° is reached, after which the blend is cooled to room temperature.
In a second phase, the silane is added and the blend is subjected to further intimate mixing, in a closed batch mixer, until a temperature of 135° C. is reached; the blend is then cooled again to room temperature.
In a final phase, the ingredients of the vulcanizing system are added and further mixing is carried out in a closed batch mixer, without exceeding a temperature of 110° C.
It should be noted that the final properties of a ready-to-use compound, and consequently the quality of the finished product, depend not only on its formula, but also, to a large extent, on the consistency of the properties of the ingredients used, which can vary from one batch to another, and on the consistency of the particular processing carried out, identified by the process parameter values, which can themselves vary randomly during processing of the blend.
For this reason, a high-quality compound produced by processes which use batch mixers is obtained by first producing a test compound and checking the properties of a number of samples of the said compound after vulcanization and then, in the event of one or more of these properties being unacceptable, correcting the values of the various process parameters as and when necessary by trial and error, until the desired result is achieved.
After the values of the various process parameters have been preset in the abovementioned manner, the consistency of the properties of the compound produced, which is necessary in order to ensure that the product has the desired requirements, is ensured by carrying out repeated checks on the properties both of the blend and of the final compound.
From the Applicant's point of view, the problem to be solved is now that of ensuring the reproducibility of the properties of the approved compound, for all the identical compounds subsequently produced, batch after batch.
Currently, before approving the compound for subsequent use and authorizing the production of a new batch, a range of checks on the mixture's physicomechanical properties are carried out; this involves long waiting times before the results of the tests are known and the risk of having produced large amounts of unsuitable material, which will have to be discarded, before being able to ascertain this unsuitability.
In particular, evaluation of the correct result for the processing as regards compounds comprising silica and silane requires a large number of controls on both the raw and vulcanized compound.
The reason for this, it should be observed, is that in order to be able to be used in a tyre, the final compound needs to show a uniform dispersion of the silica filler in the polymer base, which can be obtained during the processing of the blend in the batch mixer, and needs to have undergone a correct chemical reaction between the silica and the silane.
The quality and variability of the product which is vulcanized, with the silica-charged compounds and for each specific compound formulation, depends substantially on the embodiment of these process phases, which are respectively referred to hereinbelow by the terms “silicization” and “silanization”, which cannot be encountered in the processing of other polymers to which are added various ingredients and reinforcing fillers other than silica, for example conventional carbon black fillers.
By carrying out systematic checks on the compounds, it has been found that the compound properties considered acceptable can be within a wide range of values; in essence, a wide variability of these values has been encountered, such that it is possible for two compounds, nominally identical but manufactured one after the other, though performing satisfactorily in use, nevertheless to have properties that are very different from each other.
It should furthermore be noted that the fundamental requirement for tyres of each particular type is the reproducibility of the performance characteristics. Purely by way of example, the market specifically asks for tyres with uniform “handling” performance characteristics, which depends to a large extent on the slip force developed by the tyre.
The slip force depends, among other things, on the dynamic modulus of the tread compound, the value of which is greatly influenced by the variability of the process parameters which control the abovementioned phases of “silicization” and “silanization”, respectively.
Unfortunately, as has been stated, the process of the prior art, based on post-checking of the compounds produced, does not make it possible to achieve, as would be desired, in particular for the compounds towards which the present invention is specifically directed, high uniformity of performance characteristics between products incorporating these compounds, manufactured one after the other, unless there is a narrowing of the accepted tolerance range, and thus an increased selection of the compounds produced.